Liquid Crystal (LC) cells are used along with other optical elements in optical communication apparatuses to switch light along alternative optical pathways. In these apparatuses, LC cells usually serve as polarization controllers. The extinction ratio (Rex) of polarization is the most important parameter for these controllers.
FIG. 1 shows the diagram of a typical LC cell 102 as may be used as a polarization controller within an optical switch. In this diagram, double-barbed arrows inscribed within circles represent p-polarized light that is polarized vertically within the plane of the page. Crosses inscribed within circles in FIG. 1 represent s-polarized light polarized perpendicularly to the plane of the page. An input light 104 having an initial linear polarization, which is assumed herein to be p-polarization, is incident upon the LC cell 102.
Upon passing through the cell 102, the light becomes output light 106. Without an applied voltage (that is, in the OFF state), the polarization of the light is switched (rotated by 90°) such that, nominally, all the output light becomes s-polarized output light 106s. However, due to imperfection of the cell, a small proportion of the light is the p-polarized output light 106p. 
When voltage is applied to the cell 102 (the ON state) shown in FIG. 1, the polarization of the output light 106 nominally remains the same as that of the input light 104. Thus, most of the light is the p-polarized light 106p. Once again, however, due to imperfection of the cell, a small proportion of the light is the s-polarized output light 106s. 
For clarity of presentation, the two output lights 106s and 106p shown in FIG. 1 are shown separated from one another. In practice, however, these two output lights overlap. The output light represented by a dashed line is the one of lesser intensity. If the liquid crystal cell 102 comprises a component within an optical switch, other components of the switch may be utilized to route the s-polarized output light 106s to a first destination and the p-polarized output light 106p to a second destination. Thus, unless the light of lesser intensity is minimized, either in the ON state or the OFF state, an undesirable portion of the light may be directed to an incorrect destination.
The extinction ratio, Rex, of the cell 102 shown in FIG. 1 is defined by
                              R          ex                =                  10          ⁢                                          ⁢                                    log              10                        ⁡                          (                                                I                  s                                                  I                  p                                            )                                                          Eq        .                                  ⁢        1            where Is is the intensity of s-polarized output light 106s and Ip is the intensity of p-polarized output light 106p. If the input light 104 is s-polarized instead of p-polarized, then the quantities Is and Ip must be interchanged with one another in Eq. 1.
As the ambient temperature increases, the birefringence of the LC material generally decreases, leading to a different Rex in the OFF state. FIGS. 2A–2B show simulated curves of Rex versus applied voltage for a Twisted Nematic LC cell with a 6.8 μm thick of LC layer. The different curves represent Rex values at different temperatures. For instance, “T20”, “T30”, etc. indicate curves calculated for respective temperatures of 20° C., 30° C., etc. Assume that the OFF state of the cell 102 corresponds to an applied voltage of 0 V. Then, when the ambient temperature increases from 20° C. to 70° C., the Rex in the OFF state increases from 22.8 dB at 20° C. to 30 dB at 50° C., and then drops to 15.5 dB at 70° C. (see FIG. 2A). FIG. 2B shows the same curves exhibited in FIG. 2A, but at a wider range of voltages. If the ON state has an applied voltage of 4 V, then, as the temperature changes, the Rex in the ON state remains constant at around −50 dB (see FIG. 2B). The minus sign indicates that the p-polarized output light 106p is dominant in ON state.
During fabrication of an LC cell, the control of parameters cannot be ideal, so the parameters of each individual cell, such as cell gap, twist angle, etc., may vary. FIGS. 3A–3B show that the cell gap variation leads to different curves of Rex versus applied voltage. FIGS. 3A–3B show graphs of the simulated curves of Rex versus applied voltage for various Twisted Nematic liquid crystal cells with different LC thickness and twist angle. The different curves in each of FIG. 3A and FIG. 3B represent Rex values for cells having different cell gaps. For instance, “d65”, “d68, “d70” and “d72” represent curves for cells having gaps of 6.5° μm, 6.8° μm, 7.0° μm and 7.2° μm, respectively. Comparison between FIGS. 3A and 3B shows that twist angle variation also leads to different curves of Rex versus applied voltage.
Because, in general, a liquid crystal cell will output some light that is polarized orthogonal to the desired dominant polarization, an undesirable portion of the output light may be directed to an incorrect destination. Further, since the extinction ratio can vary with temperature within a single cell and can vary between cells, depending upon fabrication parameters, the amount of such mis-directed light is difficult to predict. This presents a problem in reproducibly fabricating optical switches based upon liquid crystal devices and in ensuring stable operation of the switches.
Accordingly, there exists a need for an improved liquid crystal cell apparatus. The improved apparatus should be made to operate at an optimum extinction ratio, regardless of temperature variations and cell-to-cell variations. The present invention addresses such a need.